3com 900B-TPC combo, BNC (thinnet or cheapernet) on debian

So I have this computer (let's call it coax) with a 3com 900B-tpc network card in it. The network card works fine when it is connected through rj45 (utp). But the card is capable of two connection types (rj45 and BNC). I have an thinnet switch that seems to work (never tested the BNC connection but the ethernet connectors works just fine). So I asked around for a 50ohm cable hooked it up like this:

-On both device's I have a T-piece connected (pc and switch).

-Between both is a coax cable and at each end is 50ohm terminator.

The problem is that debian can't get dhcp from the server. When I plug the rj45 cable into the switch and the pc everything works fine. I guess debian is asking for a ip on the wrong connector (?). This setup is just for learning purpose. If you have an idea on how to solve the problem please let me know. I appreciate any advice on this matter. I already searched the web but its a bit hard to get info or documentation on coax or cheapernet.

I think my switch is not capable of converting from thinnet to ethernet. I installed another networkcard in a winxp pc. The win xp pc was as well unable to get an ip and I had limited or no connectivity to internet. So the hub is not good. I will do some more testing this weekend by hooking up the debian and the winxp to each other (give them both an ip) and ssh or transfer some files over samba orso.